THE AIRPLANE IN SURVEYING AND MAPPING

THE airplane, while not a product of the war, owes its present prominent place to the war, and but for the war its development would have been retarded many years. It had few practical uses from the time the Wright brothers first flew their machine at Fort Myer, Va., in 1909 until 1914. It was a plaything to amuse the holiday crowd.

The war changed this situation. The allies and the central powers almost immediately saw the great importance of the airplane in battle and the best brains and energy of the warring nations were given to the problem of making the airplane perform what a few years ago would have been considered miraculous things.

War planes were made for various purposes, which I need not enumerate. But the most important thing done from the airplane was photographing the enemies' lines to obtain many kinds of military information, such as positions of batteries and ammunition dumps, changes in trench systems, troop movements,

etc.

The same methods, with some modifications, are now being considered in connection with the mapping of extensive areas by various organizations of this country. In fact, some work has already been done and experiments are being carried on which promise excellent results.

There is so much misinformation regarding surveys and maps, that it seems appropriate for me, as the head of the oldest map-making bureau of the government, to present the mapping situation to this congress, both for your information and as a matter of record.

Surveying and mapping have long histories and the development of the methods now employed took centuries. But the method of airplane surveying has developed like a mushroom. To what extent is it applicable to our needs? This I shall endeavor to show.

In collecting data for a map those surveying methods must be adopted in any particular case that suit the requirements. If one should wish only a route map running from one village to another, it would be perfectly satisfactory to use a compass for direction and the pacing of a horse or the readings of an odometer on a wheel for the distance between the two points. But maps are usually not so simple as that.

TYPES OF MAPS

There are several types of high grade maps needed in this country. One must be made along the coasts to show the location of the actual shore line and the character of the ground immediately back of the coast in order that the navigator may be able to locate himself from topographic features along the shore, should he be driven off his course during a storm. In addition the depths of the water and all obstructions to navigation must be indicated on this map or chart, and the elevation and shape of the ground on islands and near the shore line must be shown by contours.

A second class consists of maps on which the features other than elevations are shown in their correct horizontal positions. This type of map would be practically the same as the third type where the area covered is very level like the coastal plain of Louisiana.

The third class covers maps of the interior or of large islands on which all features, cultural and natural, are located in their proper horizontal positions and contours are shown to give the elevations of the ground and the shape of the hills, ridges, valleys, etc. would be used by engineers in laying out railroads and highways, and in conducting various classes of engineering work.

This map

These three classes of maps are the ones in which we are most directly interested.

The map which shows the horizontal positions of cultural and natural features on the surface of the earth, but no contours, can be made more rapidly than the one which requires contouring. All that is needed in the former case is some method of obtaining the direction and distance between each two fea

tures in the area to be surveyed. The usual method of making such a map is by compass and chain, transit and tape, transit and stadia, or by the plane table. These methods are all very closely allied and such accuracy as may be demanded may be obtained by varying the methods used.

MAPS CONTROLLED BY FIXED POINTS

In any event there must be within the area to be surveyed, if it is a large one, a number of control stations. These control stations consist of triangulation stations placed on the highest parts of the ground or traverse stations along the roads, accurately located in latitude and longitude and accurately and substantially marked with concrete or rock in order that they may be recovered and identified by the surveyors or engineers who may wish to see them.

There are now many thousands of such stations in the United States, established principally by the Coast and Geodetic Survey, available for the fundamental control of surveys and maps. From these stations control of the same or of a lower grade of accuracy may be extended in any direction for the immediate control of topographic maps.

OVERLAPS, GAPS AND OFFSETS TO BE AVOIDED It is readily seen that without the fundamental control, which extends over the whole area of the United States, there would be great confusion. If the control in any one state is not properly coordinated and correlated with that of any other state near it, the result will be that when different topographic surveys and maps are joined there will be overlaps, gaps and offsets which cause no end of trouble and confusion to the cartographer and map maker. When there is a single system of control for the whole country we avoid this unfortunate condition.

3,000,000 SQUARE MILES, LESS THAN ONE HALF

MAPPED

There is to-day only about 40 per cent. of the 3,000,000 square miles of the United States mapped both as to horizontal positions of the